EPA SW-846 Method 8151 has typically been the method of choice for the analysis of chlorophenoxy acid herbicides in groundwater and wastewater. The instrumental portion of the analysis uses gas chromatography with electron capture detection (GC/ECD). This technique has the advantage of superior sensitivity for the chlorophenoxy acid herbicides. However, because the detection by ECD is selective, not specific, the method suffers from a tendency towards false positives and elevated limits when presented with a difficult matrix. This paper will discuss an approach using gas chromatography with mass spectrometry (GC/MS) operated in the selective ion monitoring (SIM) mode that achieves the sensitivity of GC/ECD but is not affected by the same sample matrix effects as GC/ECD.